This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods that employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply xe2x80x9ccarbonylationxe2x80x9d for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy Group VIII metal; i.e., a Group VIII metal having an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof. Palladium catalysts have been found particularly useful; they include complexes with phosphines such as triphenylphosphine.
The production of carbonates may frequently be improved by including a metal-based cocatalyst along with the heavy Group VIII metal catalyst. Metal-based cocatalysts have been described broadly in U.S. Pat. Nos. 4,187,242, 4,201,721 and 5,380,907. Lead compounds as cocatalysts are particularly detailed in U.S. Pat. No. 5,498,789. Also preferred in general is the use of various halides, as illustrated by tetra-n-butylammonium bromide. Compounds characterized as inert solvents, such as toluene, diethyl ether, diphenyl ether and acetonitrile, can also be present.
Many of the catalyst systems known in the art have disadvantages such as low active catalyst lifetime, typically 2 hours or less, and low selectivity to the desired diaryl carbonate as a result of formation of relatively high proportions of by-products such as bromophenols.
Also, it has been observed that certain palladium-based catalysts, such as palladium(II) acetate, show a decrease in catalytic activity upon storage in contact with hydroxyaromatic compounds such as phenol at temperatures on the order of 70xc2x0 C. for periods as short as 2 hours. This decrease is notable particularly when the palladium compound is present in catalyst mixtures containing lead(II) oxide.
It is of interest, therefore, to develop catalyst systems that have long lifetimes, not decreased by storage, and which improve selectivity.
The present invention is based on the discovery that the presence of bisphosphines in carbonylation catalyst systems, whether added separately or as a preformed complex with the Group VIII metal, affords a catalyst with good activity and relatively long storage stability.
In one of its aspects, therefore, the invention is directed to a method for preparing a diaryl carbonate. An embodiment of the method comprises contacting at least one hydroxyaromatic compound with oxygen and carbon monoxide in the presence of a catalytic amount of a catalyst composition comprising at least one organic bisphosphine and the following optional components and any reaction products thereof: a Group VIII metal having an atomic number of at least 44 or a compound thereof; at least one bromide or chloride salt; and at least one cocatalyst which is a compound of a metal other than a Group VIII metal having an atomic number of at least 44.
Another aspect of the invention is catalyst compositions comprising the aforementioned components and any reaction products thereof.
Any hydroxyaromatic compound may be employed in the method of the present invention. Monohydroxyaromatic compounds, such as phenol, the cresols, the xylenols and p-cumylphenol, are generally preferred with phenol being most preferred. The invention may, however, also be employed with dihydroxyaromatic compounds such as resorcinol, hydroquinone and 2,2-bis(4-hydroxyphenyl)propane or xe2x80x9cbisphenol Axe2x80x9d, whereupon the products are polycarbonate oligomers.
Other reagents in the diaryl carbonate preparation method of the invention are oxygen and carbon monoxide, which react with the phenol to form the desired diaryl carbonate. They may be employed in high purity form or diluted with another gas such as nitrogen, argon or carbon dioxide, which has no negative effect on the reaction.
For the sake of brevity, the constituents of the catalyst system of the invention are defined as xe2x80x9ccomponentsxe2x80x9d irrespective of whether a reaction between said constituents occurs before or during the carbonylation reaction. Thus, the catalyst system may include said components and any reaction products thereof.
Component A of the catalyst system is one of the heavy Group VIII metals, preferably palladium, or a compound thereof. Thus, useful palladium materials include elemental palladium-containing entities such as palladium black, palladium/carbon, palladium/alumina and palladium/silica; palladium compounds such as palladium chloride, palladium bromide, palladium iodide, palladium sulfate, palladium nitrate, palladium acetate and palladium 2,4-pentanedionate; and palladium-containing complexes involving such compounds as carbon monoxide, amines, nitrites, phosphines and olefins. Preferred in many instances are palladium(II) salts of organic acids, most often C2-6 aliphatic carboxylic acids, and palladium(II) salts of xcex2-diketones. Palladium(II) acetate and palladium(II) 2,4-pentanedionate are generally most preferred. Mixtures of the aforementioned palladium materials are also contemplated.
Component B is at least one bromide or chloride salt. It may be an alkali metal or alkaline earth metal halide, preferably a bromide such as lithium bromide, sodium bromide, potassium bromide, calcium bromide or magnesium bromide. It may also be a quaternary ammonium or quaternary phosphonium salt such as tetramethylammonium bromide, tetraethylammonium bromide, tetra-n-butylammonium bromide or tetramethylphosphonium bromide, or a hexaalkylguanidinium salt such as hexaethylguanidinium bromide.
Component C is at least one organic bisphosphine. By xe2x80x9corganicxe2x80x9d is meant a compound containing at least one organic radical, with the proviso that said compound may also contain non-organic atoms or radicals. Thus, the bisphosphine may often be characterized by the formula
(R1)2P-R2-P(R1)2,xe2x80x83xe2x80x83(I) 
wherein each R1 is independently a monovalent organic radical and R2 is a divalent organic radical. Most often, R1 is an aromatic or alicyclic radical, preferably aromatic and most preferably phenyl.
The identity of the divalent R2 radical is subject to wide variation. It may be aliphatic, as exemplified by ethylene, trimethylene, tetramethylene and neopentylene. It may also be aromatic, as illustrated by phenylene and naphthylene. Suitable radicals include those containing inorganic elements, as illustrated by aminobis(alkylene) and ferrocenylene. For the most part, aliphatic radicals are preferred and C3-8 aliphatic radicals especially preferred.
Many bisphosphines of formula I, particularly the ones in which R2 is aliphatic, are commercially available; examples are 1,3-bis(diphenylphosphino)propane and 1,4-bis(diphenylphosphino)butane. Other bisphosphines can be prepared by art-recognized methods.
Bisphosphines are included in the carbonylation catalyst system in catalytic amounts. In this context a xe2x80x9ccatalytic amountxe2x80x9d is an amount of bisphosphine (or combination of bisphosphines) that increases the number of moles of diaryl carbonate produced per mole of Group VIII metal utilized; increases the number of moles of diaryl carbonate produced per mole of halide utilized; or increases selectivity toward diaryl carbonate production beyond that obtained in the absence of the bisphosphine (or combination of bisphosphines). Optimum amounts of a bisphosphine in a given application will depend on various factors, such as the identity of reactants and reaction conditions.
It is within the scope of the invention to introduce component C, the bisphosphine, into the catalyst mixture as a discrete compound. It is also contemplated to preform a complex of the bisphosphine with the Group VIII metal of component A, whereupon components A and C are introduced as a single entity. This may be achieved, for example, by a ligand interchange reaction between the bisphosphine and a palladium(II) halide complex with another ligand such as acetonitrile.
The preparation of a palladium(II) bisphosphine complex is illustrated by the following example.